The present invention relates to a photoelectric displacement measuring device.
In recent years, the development of measurement instruments in the area of length and displacement measurement has progressed enormously. For example, measurement devices have been developed for process technology and testing purposes. These devices are based upon the application of light, magnetism and the like through the use of electronic circuitry. Devices, which make use of light for measuring, are known as lightwave-interference measurement devices in which the wave-length of laser beams is drawn upon as reference magnitudes. The high precision of these measurement devices fulfills the demands of today's industrial technology to a sufficient degree, but in many cases, this high degree of accuracy requires a substantial economic outlay.
As an example of a measurement device in which the property of magnetism is exploited, GB-PS 1 270 875 discloses a magnetic measurement system in which a magnetic measurement is first recorded on a band-like magnetic element as the reference size in order to determine the relative position between this magnetic sample and a magnetic head. However, in this system, the accuracy is determined by the fineness of the magnetic differences which can be recorded on the magnetic element with a division of approximately 0.2 mm. Through interpolation of the measurement signals, one achieves a resolution of approximately 5 .mu.m-10 .mu.m, so that the accuracy of the measurement is about two orders of magnitude worse than in the case of a lightwave-interference measurement device, which can achieve a resolution of approximately 0.1 .mu.m. Thus, for example, in the case of a machine tool, a measurement device is required whose average accuracy lies between the accuracy of a lightwave-interference measurement device and the accuracy of a magnetic measurement device, so that an optical diffraction grid can be utilized whose grid constant lies in the order of magnitude of a few micrometers. Such a measurement device represents a compromise between required accuracy and justifiable costs. These types of devices, and their operation are described in DE-OS 33 16 144 and JP-OS 59-164 914.
In these types of devices, the diffraction grid constitutes the reference medium. A diffraction grid consists of very thin grid lines placed quite close together on a glass or metal plate. The grid lines may be formed by mechanical processing, an optical lithographic process, electron beam lithography or some similar process. The devices further comprise: a light source which emits monochromatic light, for example, a laser light, and a detector which receives the interference light located on the same side of the grid, and two reflector mirrors opposite of the light source on the other side of the grid. The beam of light emitted by the light source is diffracted by the diffraction grid and allowed to pass through. A light beam diffracted by the diffraction grid represents diffracted light (a diffracted light bundle) of the Nth degree, and under the influence of the diffraction grid, a value N.xi. in the wave front of the light is produced, which is the product of the degree number and the phase. A light beam, however, which passes in a straight line through the diffraction grid, does not contain any phase information. Both light beams are reflected by the reflector mirrors and return along their outbound path in order to reenter the diffraction grid and once again be diffracted and pass through. The light of the straight beam which has passed through the grid and the light diffracted to the N-th degree are spatially selected, interfere with one another, and strike a detector. The phase of the diffraction grid imparts a value -N.xi. to the second diffracted light, while the first straight light beam has a value of N.xi., so that when the two light beams interfere, a value of 2N.xi. is obtained, the value being double the amount of the phase of the diffraction grid. If one therefore assumes that the diffraction grid is moved relative to another part of the optical system, for example, relative to the light source and the reflector mirrors, then the interference light moves across 2N periods while the diffraction grid moves across one period.
In another known arrangement where a semi-transparent mirror or the like is employed, the light beam emitted by the light source is diffracted by the diffraction grid, and light bundles of the same order but with differing signs overlap and interfere with each other, prior to entering the detector. In this case, one obtains values N.xi. and -N.xi. due to the phase of the diffraction grid in the diffracted light beams, whereby N is the diffraction order number, so that one receives the interference light 2N.xi. or, in other words, an amount which is twice as large as the phase of the diffraction grid. Thus, if one once again assumes that the diffraction grid and some other part of the optical system move relative to one another, as already explained above, the interference light moves across 2N periods while the diffraction grid moves across one period.
In order to be able to accommodate the described arrangement in small areas, it is necessary to compensate angles of the light beams relative to the diffraction grid. If, however, the relative position of the optical system with regard to the diffraction grid is shifted in the direction of the grid lines of the diffraction grid, a phase change takes place, which is similar to that phase change that occurs when the relative movement occurs perpendicular to the plane of the diffraction grid, so that measurement accuracy declines. If the light beam enters vertically, the disadvantage described above will be avoided, however, the optical system will be quite extensive and thus relatively large amounts of space must be provided.
It is therefore an object of the present invention to avoid the above-mentioned disadvantage and create a displacement measurement device that is simple in construction and which is generally capable of excluding disturbances caused by ambient influences thereby assuring a reliable mode of operation. Other objects of the present invention will become apparent from the following description.